Microwave synthesis of lithium thiophosphate composite materials

ABSTRACT

A microwave induced solvothermal method to prepare lithium thiophosphate composites including α-Li 3 PS 4  and crystalline Li 7 P 3 S 11  is provided. The method is scaleable to commercial size production.

FIELD OF THE DISCLOSURE

This disclosure is directed to a method to prepare Lithium thiophosphatecomposite materials which is useful as an industrial scale process interms of product quality, operation time and energy requirement.

BACKGROUND

The ubiquitous Li-ion battery has become an integral part of society,which enabled is the revolution of portable electronic devices, notablycell phones and laptops. The next epoch will be the integration ofbatteries into the transportation and grid storage industries, furtherintensifying society's dependence on batteries. State-of-the-art Li-ioncells utilize a liquid electrolyte consisting of lithiumhexafluorophosphate salt dissolved in carbonate-based organic solvents.Recently it has become more evident that inorganic solid electrolytesare a superior alternative to liquid electrolytes which are flammableand present environmental issues.

Replacing the flammable organic liquid electrolyte with a solid Li-ionconductive phase would alleviate this safety issue, and may provideadditional advantages such as improved mechanical and thermal stability.A primary function of the solid Li-ion conductive phase, usually calledsolid Li-ion conductor or solid state electrolyte, is to conduct Li⁺ions from the anode side to the cathode side during discharge and fromthe cathode side to the anode side during charge while blocking thedirect transport of electrons between electrodes within the battery.

Moreover, lithium batteries constructed with nonaqueous electrolytes areknown to form dendritic lithium metal structures projecting from theanode to the cathode over repeated discharge and charge cycles. If andwhen such a dendrite structure projects to the cathode and shorts thebattery energy is rapidly released and may initiate ignition of theorganic solvent of the nonaqueous electrolyte.

Therefore, there is much interest and effort focused on the discovery ofnew solid Li-ion conducting materials which would lead to an all solidstate lithium battery. Studies in the past decades have focused mainlyon ionically conducting oxides such as for example, LISICON(Li₁₄ZnGe₄O₁₆), NASICON(Li_(1.3)Al_(0.3)Ti_(1.7)(PO₄)₃), perovskite (forexample, La_(0.5)Li_(0.5)TiO₃), garnet (Li₇La₃Zr₂O₁₂), LiPON (forexample, Li_(2.88)PO_(3.73)N_(0.14)) and sulfides, such as, for example,Li₃PS₄, Li₇P₃S₁₁ and LGPS (Li₁₀GeP₂S₁₂).

Generally, lithium composite sulfides tend to provide better ionicconductivity and malleability. The structural characteristics ofeffective Li⁺ conducting crystal lattices have been described by Cederet al. (Nature Materials, 14, 2015, 1026-1031) in regard to known Li⁺ion conductors Li₁₀GeP₂S₁₂ and Li₇P₃S₁₁, where the sulfur sublattice ofboth materials was shown to very closely match a bcc lattice structure.Further, Li⁺ ion hopping across adjacent tetrahedral coordinated Li⁺lattice sites was indicated to offer a path of lowest activation energy.However, their utility has been hampered by their known air-sensitivity.Presently, lithium thiophosphates (LTP) are being investigated for useas a non-volatile and thermally stable solid electrolytes inall-solid-state lithium-ion batteries. The most notable examples oflithium thiophosphate solid electrolytes include Li₃PS₄, Li₇P₃S₁₁ andLi₁₀GeP₂S₁₁. Thermally stable solid-state electrolytes allow for aparadigm shift in battery pack design by simplifying thermal managementand allowing bipolar stacking, thereby dramatically improving the energydensity beyond what would be possible for a Li-ion battery containingliquid electrolyte.

Conventionally, LTP electrolytes are synthesized by mechanochemicalmilling (i.e. ball milling) or melt quenching. As a synthetic approach,ball milling is tedious and it is debatable whether it can be costeffectively scaled up for bulk-scale synthesis. For example, to createamorphous Li₃PS₄ (i.e. a-Li₃PS₄), Li₂S and P₂S₅ powders are loaded intoa stainless steel enclosure filled with stainless steel balls and shakenor rotated for 3 days before sieving the product and cleaning the steelenclosure and balls. This method can also produce large, non-uniformparticle sizes, requiring subsequent pulverization to obtain the desiredparticle size distribution. From a manufacturing point of view,scaling-up this technology would be labor and energy intensive. Solutionsynthesis of LTP is an alternative that offers numerous potentialadvantages, but it typically takes 1-3 days, which saves no time incomparison with mechanochemical

Conventionally, a-Li₃PS₄ is synthesized by mechanochemical milling a 3:1molar ratio of Li₂S and P₂S₅ powders for 72 hours. The amorphous phaseis desirable over the crystalline phases (α, β, γ) due to its enhancedconductivity. For example, at 25° C. a-Li₃PS₄ has a conductivity ofabout 0.1 mS/cm while crystalline β-Li₃PS₄ has a conductivity of about0.001 mS/cm.

Accordingly, an object of this application is to provide a method toprepare a range of lithium thiophosphate composite materials which issuitable for product provision on an industrial scale.

Another object is to provide a method for the production of amorphousLi₃PS₄.

Another object is to provide a method for the production of Li₇P₃S₁₁.

SUMMARY OF THE EMBODIMENTS

These and other objects are provided by the embodiments of the presentapplication, the first embodiment of which includes A method to preparea target lithium thiophosphate composite, comprising: preparing ananhydrous mixture comprising: Li₂S; P₂S₅; optionally a component B; anda nonaqueous polar solvent; protecting the anhydrous mixture from airand humidity; mixing the anhydrous mixture to at least partiallydissolve the Li₂S, P₂S₅ and component B if present; applying microwaveenergy to the protected anhydrous mixture to raise the reactiontemperature to an optimum value for synthesis of the target lithiumthiophosphate composite to obtain the target lithium thiophosphatecomposite; and removing the polar solvent from the obtained lithiumthiophosphate composite; wherein a molar ratio of Li₂S to P₂S₅ and to B,if present, is determined by the composition of the target lithiumthiophosphate composite.

In an aspect of the first embodiment, when the component B is presentthe component B is at least one compound selected from the groupconsisting of Li₃N, P₂O₅ Li₂O, LiN₃, GeS₂ or LiX wherein X is I, Cl orBr.

In another aspect of the first embodiment, the nonaqueous polar solventis selected from the group consisting of ethers, nitriles, alcohols,carbonates and esters, with the proviso that the polar solvent isvolatile under reduced pressure at a temperature less than a phasetransition temperature of the target lithium thiophosphate composite.

In a special aspect of the first embodiment, the target lithiumthiophosphate composite is amorphous a ratio of Li₂S/P₂S₅ isapproximately 3/1, the solvent is tetrahydrofuran and no B component ispresent.

In another special aspect of the first embodiment, the targetlithiumthiophosphate composite is Li₇P₃S₁₁, a ratio of Li₂S/P₂S₅ isapproximately 1.05/0.45, the solvent is acetonitrile and no B componentis present.

In a more general aspect of the first embodiment the target lithiumthiophosphate composite is xLi₂S.yP₂S₅.(100-x-y)B, a ratio of Li₂S/P₂S₅is approximately x/y, and a B component is present in an amount of100-x-y.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows Raman spectra of 3Li₂S:P₂S₅ samples microwaved in varioussolvents.

FIG. 2 shows Raman spectra of the insoluble product of a microwavedreaction of a 3:1 molar ratio of Li₂S to P₂S₅ in THF.

FIG. 3 shows a TGA curve of microwaved Li₃PS₄ synthesized in THF.

FIG. 4: SEM image of microwaved. Li₃PS₄.nTHF.

FIG. 5 shows XRD patterns of amorphous (a-, grey line) and crystalline(β-, black line) Li₃PS₄.

FIG. 6 shows an Arrhenius plot of the Li⁺-conductivity of a-Li₃PS₄,measured using EIS.

FIG. 7 shows the cycling behavior of a Li/a-Li₃PS₄/Li cell at 0.1 mA/cm²with 2 h half-cycles for 20 cycles at 25° C. using microwaved α-Li₃PS₄.

FIG. 8 shows a Raman spectra of powders obtained from a reaction of a7:3 molar ratio of Li₂S to P₂S₅.

FIG. 9 shows a TGA curve of microwaved Li₇P₃S₁₁ pre-cursor synthesizedin THF.

FIG. 10 shows Raman spectra of powders obtained from a reaction of a 7:3molar ratio of Li₂S to P₂S₅.

FIG. 11 shows the TGA curves of Li₇P₃S₁₁ pre-cursors coordinated withACN.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout this description, the terms “electrochemical cell” and“battery” may be employed interchangeably unless the context of thedescription clearly distinguishes an electrochemical cell from abattery. Further the terms “solid-state electrolyte” and “solid-stateion conductor” may be employed interchangeably unless explicitlyspecified differently. The term “approximately” when associated with anumerical value conveys a range from −10% of the base value to +10% ofthe base value.

In general the inventors are conducting ongoing investigation of lithiumthiophosphate composites of formula (I):

xLi₂S.yP₂S₅.(100-x-y)B

wherein B is a composite material selected from the group of materialsincluding Li₃N, P₂O₅ Li₂O, LiN₃, GeS₂ or LiX wherein X is I, Cl or Br,each of x and y represent a mass % value of from 33.3% to 50% such thatthe total mass % of Li₂S, P₂S₅ and B is 100%. Lithium thiophosphatecompounds of interest also include Li₃PS₄, Li₇P₃S₁₁ and Li₁₀GeP₂S₁₁. Asdescribed in the background discussion of this disclosure known methodsto prepare these lithium thiophosphate composites are lengthy, often donot yield high quality product and are not scaleable to industrialcommercial batch size. Therefore, the inventors have undertaken a studyof alternative methods of synthesis of these target compounds. In thecourse of this study, microwave synthesis was investigated.

Microwave irradiation has been successfully applied in the synthesis ofboth inorganic and organic materials. For example, Kawaji et al. (US2016/0181657) describe the solid state synthesis of composite materialsbased on Li₄Sn₃O₈ doped with one or more off +2, +3, +4 and +5 metals.In comparison with conventional heating methods, reactions heated bymicrowaves may produce higher yields with milder reaction conditions andpossibly shorter reaction times. Microwaves have a high-frequencyelectric field, which oscillates polar molecules and charged species,generating heat very quickly through friction. Additionally, heat isgenerated uniformly throughout the reaction vessel as opposed to flowingtoward the reaction site via convection or conduction.

In the case of Li⁺-conductive LTP solid electrolytes, the inventorsconsidered that the Li₂S reactant is a salt comprised of Li⁺ and S²⁻ions that can interact strongly with the electric field of themicrowaves to produce heat locally. Moreover, because microwave reactionvessels are sealed, if a solvent is present, the solvent may be heatedbeyond its boiling point, which could potentially increase thesolubility of reactants including Li₂S and P₂S₅ in the solvent.

The inventors have studied the synthesis of amorphous Li₃PS₄ (α-Li₃PS₄,)and of crystalline Li₇P₃S₁₁ as described in the examples. Unexpectedly,solid state synthesis methods such as described by Kawaji et al. did notachieve the target results. However, it was surprisingly discovered thatwhen the microwave synthesis was conducted at significantly lowertemperatures in polar organic solvents high yield of the target productscould be obtained.

Conventionally, α-Li₃PS₄ is synthesized by mechanochemical milling a 3:1molar ratio of Li₂S and P₂S₅ powders for 72 hours. The amorphous phaseis desirable over the crystalline phases (β, γ) due to its enhancedconductivity. For example, at 25° C. α-Li₃PS₄ has a conductivity ofabout 0.1 mS/cm while crystalline β-Li₃PS₄ has a conductivity of about0.001 mS/cm. Initially, a solid-state, microwave synthesis of Li₃PS₄ bydirectly reacting Li₂S and P₂S₅ powders in the absence of solvent wasattempted (Example 1). The reaction was carried out at 290° C., to meltthe P₂S₅, for 40 minutes. Raman spectroscopy (FIG. 1) indicated thepresence of hypo-thiophosphate (P₂S₆ ⁴⁻ anion) and ortho-thiophosphate(PS₄ ³⁻ anion), but no PS₄ ³⁻ anion. The presence of crystalline Li₄P₂S₆and β-Li₃PS₄ was also confirmed by XRD.

Investigation of the influence of solvents and significantly lowerreaction temperatures in a solvothermal microwave induced reaction wasthen studied. Solvents in which Li₂S and P₂S₅ were at least partiallysoluble were selected. As indicated in FIG. 1 at reaction temperaturesof from 100° C. to 200° C. in anhydrous polar solvents—including dibutylether, glyme, tetraglyme, 1,3-dioxolane, acetonitrile andtetrahydrofuran (THF)—Raman spectroscopy shows the presence of thedesired PS₄ ³⁻ anions in the product. Surprisingly, the PS₄ ³⁻ anion wasthe sole product when the reaction was carried out in THF at 130° C. for30 min, albeit as β-Li₃PS₄ with residual Li₂S as shown in FIG. 2.Extending the reaction time to 3 h eliminated the residual Li₂S butstill produced β-Li₃PS₄ (FIG. 2). Thermal gravimetric analysis (TGA)showed that the THF molecules can be readily removed over a temperaturerange from 80° C. to 130° C., as shown in FIG. 3. The weight lossobtained from the TGA curve suggests a chemical composition of2Li₃PS₄.5THF. After removing the solvent at approximately 130° C. (on ahot plate) for 3 hours, the powder was pressed into a pellet, compressedbetween two stainless steel rods and the Li⁺ conductivity was measuredusing electrochemical impedance spectroscopy (EIS) at 25° C. Theconductivity of this β-Li₃PS₄ was 0.0086 mS/cm, in agreement withprevious results for β-Li₃PS₄ synthesized by ball milling.

The inventors believe that the solvent has a dramatic effect on theproduct composition, while the reaction temperature determines the phasecrystallinity. To avoid crystalline Li₃PS₄, it is necessary to performthe microwave synthesis and coordinated solvent removal at temperaturesbelow the amorphous to crystalline phase transition temperature ofLi₃PS₄. Therefore, one must be cognizant of the temperature at which thecoordinated solvent will be removed. If this temperature is too high,then the initially amorphous product will be detrimentally converted toa crystalline phase, such as β-Li₃PS₄.

Amorphous Li₃PS₄ was obtained h microwaving a 3:1 molar ratio of Li₂Sand P₂S₅ in THF at 100° C. for 3 hours (FIG. 2). In contrast to the ballmilled Li₃PS₄ sample, the microwaved sample is devoid of a P₂S₆ ⁴⁻ anionimpurity. The coordinated THF was removed at 130° C. under vacuum,resulting in a 50% weight loss. The resulting particles were eitherlarge and blocky or small as shown in FIG. 4. The inventors believe thatfurther optimization of the reaction conditions could exert more controlover particle size distribution and morphology.

FIG. 6 displays an Arrhenius plot of the Li⁺ conductivity ofmicrowave-synthesized α-Li₃PS₄ between −10° C. and 80° C. measured usingelectrochemical impedance spectroscopy (EIS), yielding values of 0.1mS/cm at 25° C. and 1 mS/cm at 80° C. The activation energy wascalculated using the Arrhenius equation, σ=σ₀e^(−E) ^(a) ^(/kT), to be160 meV. FIG. 7 shows the voltage profile of a solid stateLi/α-Li₃PS₄/Li cell as it was cycled at room temperature for 20 cyclesto demonstrate the feasibility of microwave-synthesized LTP as a Li-ionelectrolyte. During the initial cycles, the stripping and plating of Lioccurred at an overpotential of 47 mV, at 0.1 mA/cm² for 2 h eachhalf-cycle.

Crystalline Li₇P₃S₁₁ is comprised of PS₄ ³⁻ and P₂S₇ ⁴⁻ anions in a 1:1ratio. Traditionally, crystalline Li₇P₃S₁₁ is synthesized in twosteps: 1) Ball milling a 70 Li₂S: 30 P₂S₅ molar ratio of powder for 3days produces an amorphous phase of 7Li₂S-3P₂S₅ referred to as thepre-cursor. 2) Crystalline Li₇P₃S₁₁ is formed by heating the pre-cursorpowder above the crystallization temperature (˜260° C.). To demonstratethe broad applicability of the microwave solvothermal synthesistechnique, the synthesis of crystalline Li₇P₃S₁₁ was attempted.Microwave, solid-state synthesis of crystalline Li₇P₃S₁₁ (300° C., 30min) yielded a material (FIG. 8, solid line) containing P₂S₆ ⁴⁻ (385cm⁻¹) and PS₄ ³⁻ (424 cm⁻) anions but none of the desired P₂S₇ ⁴⁻ (405cm⁻¹). By using THF as a solvent and reducing the temperature to 75° C.,the desired composition of 1 P₂S₇ ⁴⁻:1 PS₄ ³⁻ may be readily obtained bymicrowaving a 70:30 molar ratio of Li₂S and P₂S₅, for 2 hours. The TGAanalysis, displayed in FIG. 9, indicates that the coordinated THF may beremoved by heating the product to 150° C., resulting in a ˜45% weightloss. This temperature is notably higher than in the case ofa-2Li₃PS₄.5THF. The dried product powder can be annealed to form thecrystalline form of Li₇P₃S₁₁.

As further example of the versatility of the microwave solvothermalmethod, Li₇P₃S₁₁ was synthesized in ACN, and the product compared withball milled Li₇P₃S₁₁ that had been soaked in ACN overnight. FIG. 10indicates that the Raman spectrum of the microwaved pre-cursor (dottedline) is similar to the soaked, ball milled Li₇P₃S₁₁ pre-cursor (dashedline). Although the Raman spectrum reveals both the P₂S₆ ⁴⁻ and PS₄ ³⁻anions, Li₇P₃S₁₁ can be formed by heating these pre-cursors to initiatea solid-state reaction between the Li₃PS₄ and Li₂P₂S₆. FIG. 11 shows theTGA analysis for both microwaved (dotted, grey line) and soaked,ball-milled (solid line) pre-cursors, demonstrating that ACN can beremoved by heating to approximately 200° C.

The inventors believe that the solvothermal microwave promoted synthesismethod may have general utility for lithium thiophosphate compositematerials and thus provide a synthesis production method suitable toproduce the target product in quantities necessary for commercial scalemanufacture in support of advanced energy generation d storage devices.

Thus, in the first embodiment, the present disclosure provides a methodto prepare a target lithium thiophosphate composite, comprising:

preparing an anhydrous mixture comprising:

-   -   Li₂S;    -   P₂S₅;    -   optionally a component B; and    -   a nonaqueous polar solvent;

protecting the anhydrous mixture from air and humidity;

mixing the anhydrous mixture to at least partially dissolve the Li₂S,P₂S₅ and component B if present;

applying microwave energy to the protected anhydrous mixture to raisethe reaction temperature to an optimum value for synthesis of the targetlithium thiophosphate composite to obtain the target lithiumthiophosphate composite; and

removing the polar solvent from the obtained lithium thiophosphatecomposite;

wherein a molar ratio of Li₂S to P₂S₅ and to B, if present, isdetermined by the composition of the target lithium thiophosphatecomposite.

The target composite lithium thiophosphate may be any composite materialas may be prepared according to the stoichiometric charge or molar ratioof the Li₂S, P₂S₅ and B if included. As indicated by the examplesdescribed herein, once the target lithium thiophosphate composite isidentified optimization of polar solvent, reaction temperature andreaction time along with other reaction variables may be achieved byroutine experimentation based on the actual composite product obtained,the product yield and the product quality.

In selected aspects of the first embodiment, B may be included as acomponent and B may be one or a combination of components selected fromLi₃N, P₂O₅ Li₂O, LiN₃, GeS₂ or LiX wherein X is I, CI or Br. Thedisclosure is not necessarily limited to only this list of B componentsand one of skill in the art may select other components as a B unit in alithium thiophosphate composite.

In one aspect of the first embodiment the target lithium thiophosphatecomposite is of formula (I):

xLi₂S.yP₂S₅.(100-x-y)B

wherein B is a composite material selected from the group of materialsincluding Li₃N, P₂O₅ Li₂O, LiN₃, GeS₂ or LiX wherein X is I, Cl or Br,each of x and y represent a mass % value of from 33.3% to 50% such thatthe total mass % of Li₂S, P₂S₅ and B is 100%.

The nonaqueous polar solvent may be any of ethers, nitriles, alcohols,carbonates and esters, with the proviso that the polar solvent isvolatile under reduced pressure at a temperature less than a phasetransition temperature of the target lithium thiophosphate composite.Generally as described in the examples the target lithium thiophosphatecomposite may be obtained as a solid precipitate in the anhydrous polarsolvent. The solid may be isolated from the mother liquor by any methodknown in the art, including decantation, filtration and centrifugation.

The solid material obtained from the isolation may contain coordinatedsolvent molecules which may be removed to obtain the solvent-freecomposite material. The coordinated solvent may be removed by any methodknown for such purpose and may require heating the material to anelevated temperature, preferably under reduced pressure, more preferablyunder vacuum. As previously described the temperature at which thesolvent is removed must be at a value less than a phase transitiontemperature associated with the target lithium thiophosphate composite.

In one special embodiment of the present disclosure, the target lithiumthiophosphate composite is amorphous Li₃PS₄ (α-Li₃PS₄), a ratio ofLi₂S/P₂S₅ is approximately 3/1, the solvent is tetrahydrofuran and no Bcomponent is present. As described in the examples the temperature ofthe microwave induced solvothermal reaction may be conducted at atemperature of approximately 100° C. or less for a reaction time ofapproximately 30 minutes to 3 hours. The product α-Li₃PS₄ may beobtained by isolation from the mother liquor portion of the THF and thenremoval of the THF coordinated with the α-Li₃PS₄.

In another special embodiment of the present disclosure, the targetlithium thiophosphate composite is Li₇P₃S₁₁, a ratio of Li₂S/P₂S₅ isapproximately 70/30, the solvent is THF or acetonitrile and no Bcomponent is present. As described in the examples the temperature ofthe microwave induced solvothermal reaction may be conducted at atemperature of approximately 75° C. or less for a reaction time ofapproximately 1 to 3 hours. The precursor product to Li₇P₃S₁₁ may beobtained by isolation from the mother liquor portion of the THF oracetonitrile and then removal of the coordinated solvent. Li₇P₃S₁₁ maythen be obtained by annealing to form the crystalline Li₇P₃S₁₁.

In a more general embodiment of the present disclosure the targetlithium thiophosphate composite is xLi₂S.yP₂S₅.(100)x-y)B, a ratio ofLi₂S/P₂S₅ is approximately x/y, and a B component is present in anamount of 100-x-y. The anhydrous polar solvent, temperature of themicrowave induced reaction and time of the reaction may be determined byroutine experimentation. Generally the anhydrous polar solvent may beselected from ethers, nitriles, alcohols, carbonates and esters, withthe proviso that the polar solvent is volatile under reduced pressure ata temperature less than a phase transition temperature of the targetlithium thiophosphate composite. Generally, the time of the microwaveinduced reaction may be from 30 minutes to 5 hours, preferably from 30minutes to 4 hours and most preferable from 30 minutes to 3 hours.Generally, the reaction temperature may be from 50° C. to 200° C.,preferably 70° C. to 150° C. and most preferably from 75° C. to 130° C.The target lithium thiophosphate composite may be isolated from themother liquor by methods previously described and freed of coordinatedsolvent as previously described.

The above description is presented to enable a person skilled in the artto make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein. In thisregard, certain embodiments within the invention may not show everybenefit of the invention, considered broadly.

EXAMPLES Example 1 Attempted Microwave Solid State Synthesis of a-Li₃PS₄

A solid-state, microwave synthesis of Li₃PS₄ by directly reacting Li₂Sand P₂S₅ powders in the absence of solvent was attempted. The reactionwas carried out at 290° C., to melt the P₂S₅, for 40 minutes. Ramanspectroscopy was employed to characterize the local structure of theproduct, which is shown in FIG. 1. The peaks at 385 cm⁻¹ and 422 cm⁻¹are assigned to the hypo-thiophosphate (P₂S₆ ⁴⁻ anion) andortho-thiophosphate (PS₄ ³⁻ anion), respectively. The presence ofcrystalline Li₄P₂S₆ and β-Li₃PS₄ was also confirmed by XRD.Specifically, the presence of the (221) peak but absence of the (211)peak confirms that the product is β-, and not γ-, Li₃PS₄.

Example 2 Attempted Microwave Solvothermal Synthesis of a-Li₃PS₄

To reduce the temperature of the reaction, we investigated solvents inwhich Li₂S and P₂S₅ were partially soluble. In a wide variety ofanhydrous polar solvents—including dibutyl ether, glyme, tetraglyme,1,3-dioxolane, acetonitrile and THF—Raman spectroscopy once againreveals the presence of the desired PS₄ ³⁻ anions in the product (FIG.1), similar to the solvent-free reaction. In FIG. 1 the Raman spectra of3Li₂S:P₂S₅ samples microwaved in various solvents, includingacetonitrile (top, 1^(st)), 1,3 dioxolane (2^(nd)), tetraglyme (3^(rd)),glyme (4^(th)), dibutyl ether (5^(th)) and tetrahydrofuran (THF, 6^(th))are shown. The Raman spectrum of pure Li₂S (solid, bottom) is also shownto demonstrate that some of the microwave syntheses did not react tocompletion in these cases. The PS₄ ³⁻ anion was the sole product whenthe reaction was carried out in THF at 130° C. for 30 min, albeit asβ-Li₃PS₄ with residual Li₂S as shown in FIG. 2.

In FIG. 2 the Raman spectra of the insoluble product of a microwavedreaction of a 3:1 molar ratio of Li₂S to P₂S₅ in THF is shown. Thelong-dashed line represents a reaction carried out at 130° C. for 30min; the short-dashed line represents a reaction carried out at 130° C.for 3 h; the dotted line represents a reaction carried out at 100° C.for 3 h. The solid line is ball milled, amorphous Li₃PS₄ for comparison.

Extending the reaction time to 3 h eliminated the residual Li₂S butstill produced β-Li₃PS₄ (FIG. 2). Thermal gravimetric analysis (TGA)showed that the THF molecules can be readily removed over a temperaturerange from 80° C. to 130° C., as shown in FIG. 3. The weight lossobtained from the TGA curve suggested a chemical composition of2Li₃PS₄.5THF. After removing the solvent at approximately 130° C. (on ahot plate) for 3 h, is the powder was pressed into a pellet, compressedbetween two stainless steel rods and the Li⁺ conductivity was measuredusing electrochemical impedance spectroscopy (EIS) at 25° C. Theconductivity of this β-Li₃PS₄ was 0.0086 mS/cm, in agreement withprevious results for β-Li₃PS₄ synthesized by ball milling.

Example 3 Synthesis of α-Li₃PS₄ (Amorphous Li₃PS₄)

Amorphous Li₃PS₄ was obtained by microwaving a 3:1 molar ratio of Li₂Sand P₂S₅ in THF at 100° C. for 3 hours (FIG. 2). In contrast to the ballmilled Li₃PS₄ sample, the microwaved sample was devoid of a P₂S₆ ⁴⁻anion impurity. The coordinated THF was removed at 130° C. under vacuum,resulting in a 50% weight loss. The resulting particles were eitherlarge and blocky or small as shown in FIG. 4. After the reaction, only awhite powder remained at the bottom of the vial and the solution wascolorless and clear.

Annealing is the conversion of the amorphous product into a partially orcompletely crystalline form. Before annealing, the absence of anyreflections (i.e. peaks) in the XRD pattern, shown in FIG. 5 (lowermostscan), inferred that the sample was amorphous. Confirmation of theLi₃PS₄ product was established by heating the powder at 150° C. for 1hour to form β-Li₃PS₄, which can be positively identified by itsfingerprint XRD pattern, as shown in FIG. 5 (uppermost scan). In FIG. 5the broad hump centered at about 21° is from the quartz capillary.

FIG. 6 shows an Arrhenius plot of the Li⁺ conductivity ofmicrowave-synthesized a-Li₃PS₄ between −10° C. and 80° C. (measuredusing EIS), yielding values of 0.1 mS/cm at 25° C. and 1 mS/cm at 80° C.The activation energy was calculated using the Arrhenius equation,σ=σ₀e^(−E) ^(a) ^(/kT), to be 160 meA. FIG. 7 shows the voltage profileof a solid state Li/α-Li₃PS₄/Li cell as it was cycled at roomtemperature at 0.1 mA/cm² with 2 h half-cycles for 20 cycles todemonstrate the feasibility of microwave-synthesized LIP as a Li-ionelectrolyte. During the initial cycles, the stripping and plating of Lioccurred at an overpotential of 47 mV, at 0.1 mA/cm² for 2 h eachhalf-cycle.

Example 4 Attempted Microwave Solid State Synthesis of Li₇P₃S₁₁

Crystalline Li₇P₃S₁₁ is comprised of PS₄ ³⁻ and P₂S₇ ⁴⁻ anions in a 1:1ratio.

Microwave, solid-state synthesis of a 70:30 molar ratio of Li₂S and P₂S₅at 300° C. for 30 min yielded a material having the Raman spectra shownin FIG. 8 (solid line) containing P₂S₆ ⁴⁻ as indicated by the Raman bandat 385 cm⁻¹ and PS₄ ³⁻ as indicated by the Raman band at 424 cm⁻¹ butnone of the desired P₂S₇ ⁴⁻ which is characterized by a Raman band at405 cm⁻¹.

Example 6 Microwave Solvothermal Synthesis of Li₇P₃S₁₁ (THF)

A 70:30 molar ratio of Li₂S and P₂S₅ was mixed in THF and microwave heated to 75° C. for 2 hours. As indicated in FIG. 8 (upper dotted curve)the desired composition of 1. P₂S₇ ⁴⁻:1 PS₄ ³⁻ was obtained. The TGAanalysis, displayed in FIG. 9, indicates that the coordinated THF can beremoved by heating the product to 150° C., resulting in a ˜45% weightloss. The dried product powder was annealed to obtain the crystallineform of Li₇P₃S₁₁.

Example 7 Microwave Solvothermal Synthesis of Li₇P₃S₁₁ (ACN)

A 70:30 molar ratio of Li₂S and P₂S₅ was mixed in acetonitrile (ACN) andmicrowave heated to 100° C. for 3 hours. As indicated in FIG. 10 (upperdotted curve) the desired. composition of 1 P₂S₇ ⁴⁻: 1 PS₄ ³⁻ wasobtained. The TGA analysis, displayed in FIG. 11, indicates that thecoordinated THF can be removed by heating the product to 150° C.,resulting in a 45% weight loss.

The solid line in FIG: 11 represents ball milled reaction of a 7:3 molarratio of Li₂S to P₂S₅. The dashed line is the product of soaking ballmilled, amorphous Li₇P₃S₁₁ in acetonitrile (ACN) for 24 hours, followedby acetonitrile removal. The dotted line represents the product of themicrowave reaction in acetonitrile described above. “P₂S₆ ⁴⁻. ACN” and“PS₄ ³⁻. ACN” (dotted lines) show how ACN coordination increased theRaman excitation energy of those vibrations.

Example 8 Detailed Synthesis of Amorphous Li₃PS₄

Into a 10 mL silicon carbide microwave vial (Anton Paar), anhydrous THF(Manchester Organics, 3 mL) and a stir bar were added. To the vial, Li₂S(Aldrich, 99.98%, 41.4 mg, 0.900 mmol) and P₂S₅ (Sigma-Aldrich, 99%,66.7 mg, 0.3 mmol) powder were added. The vial was immediately cappedand vortexed. The vial was then transferred from the glove box to themicrowave reactor. The mixture was heated to 100° C. for 3 hours with astir rate of 1200 rpm. Due to the air sensitivity of the lithiumthiophosphates, the IR temperature sensor was used to control thetemperature. After the synthesis was complete, the vial was returned tothe glove box and the white insoluble product was removed using suctionfiltration. After the solvent was removed, the residue was slurried inanhydrous heptane (10 mL) and the insoluble product was collected bysuction filtration. The coordinated solvent (i.e. THF) was removed fromthe insoluble product by heating it to 124° C. on a hot plate in theglove box for 3 hours.

Example 8 Detailed Synthesis of Amorphous Li₇P₃S₁₁ Pre-Cursor

Into a 10 mL quartz microwave vial (Anton Paar), anhydrous acetonitrile(ACN) (Manchester Organics, 3 mL) and a stir bar were added. To thevial, Li₂S (Aldrich, 99.98%, 48.2 mg, 1.05 mmol and P₂S₅ (Sigma-Aldrich,99%, 100 mg, 0.45 mmol) powder were added. The vial was immediatelycapped and vortexed. The vial was then transferred from the is glove boxto the microwave reactor. The mixture was heated to 100° C. for 3 hourswith a stir rate of 1200 rpm. Due to the air sensitivity of the lithiumthiophosphates, the IR temperature sensor was used to control thetemperature. After the synthesis was complete, the vial was returned tothe glove box and the acetonitrile was removed at 50° C. under vacuum,leaving behind a white product. After the bulk solvent was removed, thecoordinated solvent (i.e. ACN) was removed from the powder product byheating it to 200° C. in a Buchi B-585 glass oven for 1 hour.

TGA Analysis.

TGA analysis was performed within an Ar glove box (H₂O, O₂<0.1 ppm)using a Netzsch Luxx STA 409 PC. About 8 mg of sample powder was loadedinto a cold-sealable, DSC pan with a 75 μm diameter hole in the lid. TheDSC pan was crimped, loaded into the Netzsch Luxx and heated at 2°C./min. The reference was an empty, crimped DSC pan with a hole in thelid.

Raman Analysis.

Raman spectroscopy was performed with a Horiba LabRAM HR spectrometerequipped with an inverted optical microscope. A 50× lwr objective lenswas used to focus a 532 nm laser onto the powder sample, which waspressed against the inside surface of a sealed emetic to protect it fromair. The hack-scattered light was dispersed using a 600 grating/mmgrating onto a CCD camera. Spectra were collected by performing 20sequential scans, each with a 1 second duration. Spectra were collectedfrom four different spots on each sample and compared to confirm samplehomogeneity.

SEM Analysis.

SEM images were collected using a JEOL 7800 FLV at a magnification of500× with an acceleration voltage of 5 kV and a beam current of 8 (43pA).

Powder XRD Analysis.

A Rigaku SmartLab 3 kW fitted with an Anton-Paar HTK 1200N oven chamberand capillary extension was used to collect the XRD patterns. The 0.3 mmdiameter quartz capillary was filled with dried, amorphous Li₃PS₄ andsealed with epoxy in the glove box before transferring it to thediffractometer. Patterns were collected with a 0.035° step size at arate of 0.4167°/min. After the amorphous material was scanted, thecapillary was heated to 150° C. to crystallize the material intoβ-Li₃PS₄. Multiple scans were performed so that the first and finalpatterns could be compared to confirm that the pattern did not changewith time and, therefore, that the sample was successfully protectedfrom air. The repeated scans were then added together to form the finalXRD patterns.

Conductivity Measurement.

Into a Macor pellet die, 100 mg of Li₃PS₄ powder was added and thenpressed at 66.4 bar for 1 min to form an 11.28 mm diameter pellet (i.e.1.0 cm²). Then, carbon-coated aluminum foil (MTI corp.) was pressed intoboth sides of the pellet. This stack of materials was compressed atabout 88 MPa in an air-tight cell, attached to a Bio-logic VMP3potentiostat, and placed into a temperature-controlled oven.Electrochemical impedance spectroscopy was used to measure the compleximpedance of the cell at increasing temperatures from −10° C. to 80° C.

Electrochemical Cycling.

Into a Macor pellet die, 100 mg of Li₃PS₄ powder was added d thenpressed at 66.4 bar for 1 min to form an 11.28 mm diameter pellet (i.e.1.0 cm²), 596 μm thick. Then, polished and flattened lithium foil discs(99.8%, Honjo Metal) were applied to both sides of the pellet, followedby 508 μm-thick 400 nickel spacers (McMaster-Carr) and 9.5 mm diameterwave springs (McMaster-Carr). The spring/spacer/Li/solidelectrolyte/Li/spacer/spring stack was compressed to 8.8 MPa in a cell,transferred to a 25° C. oven and cycled in an air-tight container usinga Bio-logic VMP3 potentiostat. The cell was cycled galvanostatically at100 μA/cm² with 0.2 mAh/cm² half-cycles.

1. A method to prepare a target lithium thiophosphate composite,comprising: preparing anhydrous mixture comprising: Li₂S; P₂S₅;optionally a component B; and a nonaqueous polar solvent; protecting theanhydrous mixture from air and humidity; mixing the anhydrous mixture toat least partially dissolve the Li₂S, P₂S₅ and component B if present;applying microwave energy to the protected anhydrous mixture to raisethe reaction temperature to an optimum value for synthesis of the targetlithium thiophosphate composite to obtain the target lithiumthiophosphate composite; removing the polar solvent from the obtainedlithium thiophosphate composite; wherein a molar ratio of Li₂S to P₂S₅and to B, if present, is determined by the composition of the targetlithium thiophosphate composite.
 2. The method to prepare a targetlithium thiophosphate composite according to claim 1 wherein thecomponent B is present and is at least one compound selected from thegroup consisting of Li₃N, P₂O₅ Li₂O, LiN₃, GeS₂ or LiX wherein X is I,Cl or Br.
 3. The method to prepare a target lithium thiophosphatecomposite according to claim 1 wherein the nonaqueous polar solvent isselected from the group consisting of ethers, nitriles, alcohols,carbonates and esters, with the proviso that the polar solvent isvolatile wider reduced pressure at a temperature less than a phasetransition temperature of the target lithium thiophosphate composite. 4.The method to prepare a target lithium thiophosphate composite accordingto claim 1 wherein the microwave energy applied the protected anhydrousmixture to induce reaction raises the temperature to a value of from 50°C. to 200° C.
 5. The method to prepare a target lithium thiophosphatecomposite according to claim 1 wherein a time of the microwave inducedreaction may be from 30 minutes to 5 hours.
 6. The method to prepare atarget lithium thiophosphate composite according to claim 1 wherein thetarget lithium thiophosphate composite is amorphous Li₃PS₄, a ratio ofLi₂S/P₂S₅ is approximately 3/1, the solvent is tetrahydrofuran and no Bcomponent is present.
 7. The method to prepare a target lithiumthiophosphate composite according to claim 1 wherein the target lithiumthiophosphate composite is Li₇P₃S₁₁, a ratio of Li₂S/P₂S₅ isapproximately 70/30, the solvent is tetrahydrofuran or acetonitrile andno B component is present.
 8. The method to prepare a target lithiumthiophosphate composite according to claim 1 wherein the target lithiumthiophosphate composite is xLi₂.yP₂S₅.(100-x-y)B, a ratio of Li₂S/P₂S₅is approximately x/y, and a B component is present in an amount of100-x-y, and wherein the component B is at least one compound selectedfrom the group consisting of Li₃N, P₂O₅ Li₂O, LiN₃, GeS₂ or LiX whereinX is I, Cl or Br.